Method for generating vacuum in mass-exchange columns

ABSTRACT

The present invention pertains to the field of chemical and oil refining technology. In particular, the invention relates to the vacuum distillation of liquid mixtures and relates directly to the separation of liquid mixtures. The main field of application of the invention is rectification of oil stock in the petrochemical industry. The objective of this invention is to reduce power consumption required for generating a vacuum in mass-transfer columns while improving the quality of target products, reducing environmental pollution and increasing the efficiency of separation. According to the invention, a vacuum in a mass transfer column is generated due to condensation of outgoing vapors and evacuation of non-condensable gas-vapors by a vacuum-producing system, which is connected to a vacuum pump and composed of an absorber, a phase separator and a cooler, connected in series. A liquid phase (or mixture of phases) with the lowest possible solidification temperature is used as an absorbent for condensation of the gas-vapors and for producing a vacuum. This liquid phase (or mixture of phases) is pre-separated in the phase separator and is cooled after separation down to a temperature which is not lower than its solidification temperature. The introduced method is less power-intensive and more effective in comparison to the most advanced methods using vacuum pumps.

BACKGROUND OF THE INVENTION

The present invention pertains to the field of chemical and oil refiningprocesses. In particular, the invention relates to the processes ofvacuum distillation of liquid mixtures, which are used for example forfractionation of hydrocarbon liquids, and relates directly to theseparation of liquid mixtures.

The invention can be applied in the petrochemical industry for oil stockrectification as well as in chemical, food, pharmaceutical and otherindustries.

The following methods that are used in oil refining for producing avacuum are known (see, for example, “Oil technology”, the book of L. I.Gurevitch, part 1, Gostoptechizdat, 1952, pages 267-268):

A mechanical method (by dry and wet displacement and rotary pumps);

A physical method based on the instantaneous condensation of the watersteam, passing from the top of a column, in a surface or mixingcondenser;

A method based on usage of kinetic energy of an active liquid jet inejectors etc.

A combination of some of the above mentioned methods is used in themethod for vacuum processing of a fuel oil introduced in SU (A. C CCCP)No. 1447637, M. cl. C10 G 7/06, 1967.

According to this method a heated-up fuel oil is rectified under vacuumin a vacuum column. The method includes withdrawing of side-cutdistillates from the column, bleeding of non-condensable gases andvapors from the top of the column, feeding of the vapors back into thebottom of the column for additional extraction of a distillate fractionunder vacuum in the presence of non-condensable gases and vapors of thevacuum column, and condensation and absorption of the distillatefraction, extracted from the residue, by a chilled mixture of the lowerside-cut distillate of the vacuum column and distillate fraction.

An imperfection of this method is incomplete utilization of energy ofthe chilled liquid for condensation of the vapors. That is why a deepervacuum can not be obtained and the output of the distillate fractionscan not be increased.

The closest analogue of the present invention is a method for producingvacuum in mass-transfer columns, wherein suction of un-condensed gasesfrom the top of a column is effected by a chilled reflux, generated as aresult of condensation of outgoing hydrocarbon vapors. The hydrocarbonvapors pass from the mass-transfer column into surface condensers. Areflux, generated in the condensers as a result of condensation of thehydrocarbon vapors, drains into a collector. A part of the reflux is fedback into the column for refluxing. A jet pump imparts pressure energyto those parts of the reflux, which ensures suction of gases. Aftercooling in a cooler the reflux is fed into an ejector, which evacuatesgases from the condensers. A mixture of the un-condensed gases andreflux is separated in a separator. The gases are released to theatmosphere or bled for processing (or purification), the reflux passesto the collector for further delivery to the ejector by the jet pump(see SU (A. C CCCP) No. 1019645, M. cl. B01 D 3/00, 1993).

The main imperfection of the analogue method is the necessity to use thetotal amount of the cooled reflux, condensed from the outgoing vapors,for suction of non-condensable gases. This results in repeatedcontamination of the reflux by impurity components of the gases andvapors outgoing from the top of the column and consequently indeterioration of the quality of the base distillates so that additionalexpenses for depuration of the distillates are required.

SUMMARY OF THE INVENTION

The present invention is aimed at a reduction of power inputs requiredfor vacuum processing of oil derivatives, an improvement of the qualityof the base products, a reduction of environmental pollution and anincrease of separation efficiency.

The objectives of the invention are attained due to application of amethod for producing a vacuum consisting

condensation of vapors outgoing from a mass-transfer column, suction ofnon-condensable gases by a vacuum-producing system, which is connectedto a vacuum pump and is composed of an absorber, a phase separator and acooler, connected in series, separation of a liquid phase with thelowest solidification temperature in the phase separator, cooling of theliquid phase after the separation to a temperature which is not lowerthan the solidification temperature of this liquid, use of the cooledliquid phase as an absorbent for condensation of the vapors and forproducing a vacuum.

BRIEF DESCRIPTION OF DRAWINGS

The drawing in FIG. 1 represents a schematic diagram of a system thatembodies the introduced method.

DETAILED DESCRIPTION

The offered method is implemented as follows.

A stock product, for example fuel oil—the residue of the wetdistillation, in the amount of 250 tons/hour is fed through a pipeline 1into a furnace 2 for preheating. Then the flow of the stock product at atemperature of 385° C. and under a pressure of 200 mm Hg passes througha pipeline 3 into a column 4. A distillate in the amount of 120tons/hour is bled from the middle section of the column 4 through apipeline 5. A distillation residual in the amount of 130 tons/hour isbled from the bottom of the column 4 through a pipeline 6. The columnhas a circulating reflux 7 for abstraction of the excess heat ofcondensation. Water-steam in the amount of 0.5% of the stock productmass is fed to the bottom of the column through a pipeline 8.

Gases and vapors at a temperature of 120° C. and under a pressure of 26mm Hg in the total amount of 2400 kg/hour are bled from the top of thecolumn 4 through a pipeline 9 to an absorber 10. A part of the gases andvapors is condensed in the absorber 10 by a cooled absorbent. A mixtureof the absorbent and condensed part of gases and vapors at a temperatureof 5° C. passes from the absorber 10 through a pipeline 11 to a receiver12 of a phase separator 13. The receiver 12 constitutes a hydro seal.The two-phase mixture runs from the receiver 12 over a wall 14 into thezone of separation furnished with separating elements 15 (inclinedplates, for example—displayed schematically) dividing the mixture intothin layers and providing more intensive separation of phases. In thezone of separation the mixture is separated into gaseous and liquidphases. As a result of the separation the liquid phase is divided intocontinuous phase layers differing by their relative density (prevalentdemulsifiers can be used to promote the process). The gaseous phase isbled after separation through a separator 16 and a pipeline 19.

Thus, the two-phase mixture is separated in the phase separator 13 intothe following components (phases): a liquid phase No. 1 (water), aliquid phase No. 2 (hydrocarbons) and a gaseous phase No. 3, which arebled from the phase separator through pipelines 17, 18, 19 accordingly.One of the liquid phase layers (for example water) is a liquid ofopposite polarity towards another part of the liquid phase(hydrocarbons). Presence of the polar liquid accelerates separation ofthe emulsion into the phase layers.

The liquid phase No. 2 can be separated additionally into another two(or more) phase layers differing by their relative density and byfreezing point accordingly. This allows (if necessary) extraction of aphase layer No. 2¹ with different (lower, for example) freezing pointfrom the liquid phase No. 2. Therefore the absorbent with the requiredfreezing point can be selected more precisely. In case the liquid phaseis separated into the phase layers it is preferable to bleed each phaselayer separately. For example, if the freezing point of the phase layerNo. 2¹ is higher than the same of liquid phase No. 2, the phase layerNo. 2¹ is bled through a pipeline 20 (otherwise it is gainful to use thephase layer No. 2¹ as the absorbent). An excess amount of the liquidphase used as the absorbent (No. 2 for example) is bled from the systemthrough a pipeline 21. The remainder of this liquid phase is fed into acooler 22, where it is cooled down to −5° C. Then the cooled liquidphase—absorbent is fed into the top of the absorber 10 through apipeline 23. The non-condensable gas-vapors at a temperature of −4° C.and under a pressure of 28 mm Hg in the amount of 268 kg/hour areevacuated from the top of the absorber 10 by a vacuum pump 25 through avacuum pipeline 24.

Industrial applicability: The invention may be applied in oil refining,chemical and other industries, which are connected with processing ofhydrocarbon liquids as a feed stock and as a freak stock. Integration ofthe described method with the traditional process schemes offractionation in combination with the rational transfer of hydrocarbonliquids from one process position to another is possible.

The application of the offered method reduces energy inputs required forproducing vacuum because it provides a reduction of the vacuum pumpload. Besides, a deeper vacuum can be achieved due to the use of adistillate fraction (a mixture of hydrocarbons), whose freezing point isbelow zero, as the absorbent. It is possible to match the absorbent withthe required freezing point by very accurately varying the compositionof the absorbent mixture (the composition of this mixture depends on thecolumn's design—quantity of theoretical plates at the top of the column,for example—and its process parameters). The “absorber—phase separator”circulation system does not require a make up supply of liquid, as thesystem produces minimal quantities of contaminated products, which canbe bled as separate flows and recovered easily.

The offered method can be used also for the distillation of fuel oil. Inthis case it is not necessary to use liquids of opposite polarity as theevaporating agents, because, according to the method, the temperature ofthe circulating absorbent (at the outlet of the cooler 22) is alwayshigher than its freezing point.

In practice the introduced method for producing a vacuum is at least ourtimes more efficient in comparison with the most advanced methods usingvacuum pumps.

What is claimed is:
 1. A method for producing a vacuum in amass-transfer column, consisting the steps of: condensing an outgoingvapor from a mass-transfer column; suctioning a non-condensable gas by avacuum-producing system connected to a vacuum pump, the vacuum producingsystem including an absorber, a phase separator and a cooler connectedin series; separating a liquid phase with a lowest freezing temperaturein the phase separator; cooling the liquid phase after said separatingstep down to a temperature which is not lower than a solidificationtemperature of the liquid phase; and using the liquid phase after saidcooling step as an absorbent for said condensing step and for producingthe vacuum.
 2. A method for producing a vacuum in a mass-transfercolumn, consisting the steps of: condensing an outgoing vapor from amass-transfer column to form a non-condensable gas and a mixture of anabsorbent and a condensate; evacuating the non-condensable gas; passingthe mixture to a phase separator; separating a liquid phase with alowest freezing temperature in the phase separator; cooling the liquidphase after said separating step down to a temperature which is notlower than a solidification temperature of the liquid phase; and usingthe liquid phase after said cooling step as the absorbent for saidcondensing step and for producing the vacuum.